The invention relates to the treatment of brine containing sodium sulphate (Na.sub.2 SO.sub.4) and potassium chloride (KCl) content in solution. Particularly it relates to a treatment of brine elutriates after evaporation treatment to extract some of the sodium chloride (NaCl) in the brine. Such elutriates have considerable levels of sodium sulphate and potassium chloride in solution.
In the extraction of sodium chloride from natural deposits a method is used which consists of dissolving out the salt deposits by the addition of water, and of evaporating an almost saturated crude brine thus obtained following conventional purification, in order to crystallize out the dissolved sodium chloride. The crude brine produced in the dissolving step and which is to be processed usually contains further secondary components in addition to NaCl, which on the one hand may be undesirable in the crystallized product, the NaCl, and on the other hand may lead to intrusive incrustations (boiler scale) on the apparatus, heat transmission surfaces and pipe ducts in the evaporation process.
The so-called hardness-causing salts can be removed from the crude brine by chemical pretreatment, in a purification stage in which magnesium is precipitated as hydroxide and calcium as carbonate. Components of higher solubility, such as for example, sulphates and potassium compounds, remain in the purified brine solution.
To prevent the crystallization of the higher solubility components with the sodium chloride in the evaporation stage, the stage is controlled such that the concentration of the higher solubility components remains below their saturation. As a result no crystallization and no mixing with the product, the sodium chloride, takes place. To achieve this an evaporation ratio is adhered to from which a permanent residual solution or parent solution results. The parent solution contains components which are not to be crystallized, with a residual content of sodium chloride. Whether the evaporation process takes place continuously or in stages, the parent solution is drawn off as elutriate.
For sulphate the permissible threshold concentration in the above treatment is dependent on the operation temperature; e.g., 4.5% at 100.degree. C. approximately, and 5.2% at 5O.degree. C. approximately, as Na.sub.2 SO.sub.4 at the same time in addition approximately 26% or 24% NaCl respectively will be in solution.
The parent solution, or brine elutriate, is drained off in more simple methods or conveyed to another use. According to known methods the salting out effect is used which, by the admixture of easily soluble sulphates leads to precpitation of CaSO.sub.4. For this purpose, the parent solution which is almost saturated in Na.sub.2 SO.sub.4 can be mixed with the crude brine which is to be purified and a salting-out effect, with precipitate of CaSO.sub.4, is brought about.
Insofar as in the above treatment of the elutriate the sulphate content in the crude brine is contained to the extent of the calcium sulphate solubility, a balanced condition will arise between brine purification, evaporation and elutriate, and the available brine can be practically fully utilized, in which the brine elutriate is returned and is subjected once more to evaporation. However, if the sulphate content in the crude brine is higher; i.e., above the calcium sulphate solubility, then no equalized balance will arise and a part of the sulphate must be removed from the system, either by discarding a part of the elutriate brine, or by precipitation of a sulphate content by cooling in the range of the Glauber's salt phase; i.e., for example to 0.degree. C., or by chemical treatment, for example calcium chloride (CaCl.sub.2)--admixture. The balance of the sulphate introduced with the crude brine and elutriate must be equalized.
If the crude brine also contains a proportion of potassium chloride (KCl), then this component will accumulate in the above methods and finally will crystallize out with the NaCl and may be considered as an undesired constituent part. To prevent this, a part of the elutriate is discarded in a quantity such that the balance of introduced and elutriated potassium is equalized.
In order to avoid the above-mentioned loss of NaCl in the elutriate brine which is to be discarded, known methods consist of precipitating out the sodium sulphate for the most part through the addition of calcium chloride (CaCl.sub.2) and through evaporation at a high temperature (e.g. 120.degree. C.), and again crystallizing out NaCl until almost the saturation of KCl is reached. Additionally, as is known from the potassium industry, by temperature lowering KCl can be crystallized out and thereafter once more NaCl can be crystallized out by further evaporation in the higher temperature range. Whatever the case may be, in the end a residual quantity, which is again enriched with sodium sulphate, is either discarded or, if required, may be returned into the brine purification process. The disadvantage of this method lies in that an additional chemical treatment is necessary, requiring calcium and sodium carbonate and corresponding apparatus.